The present invention relates to a platen used as a stator of a planar linear motor, more particularly relates to a platen comprised of a plurality of magnetic sheets.
First, explaining the principle of a Sawyer linear motor, as shown in FIG. 30, it is comprised of a platen (stator) 10 comprised of a magnetic thick plate on whose surface is repeatedly formed platen dots D at a spatial period of the dot pitch P and a movable member (traveling member) 20 comprised of a permanent magnet M for generating a bias magnetic flux, first and second yokes Y1 (Y2) bonded to the magnetic pole surface to be arranged in parallel to the direction of advance and provided with first and second branched magnetic path legs A and Axe2x80x2 (B and Bxe2x80x2), series-connected first and second A-phase excitation coils CA and CAxe2x80x2 wound around the first and second branched magnetic path legs A and Axe2x80x2 of the first yoke Y1, series-connected first and second B-phase excitation coils CB and CBxe2x80x2 wound around the first and second branched magnetic path legs B and Bxe2x80x2 of the second yoke Y2, and two pole teeth (projecting poles) KA and KAxe2x80x2 (KB and KBxe2x80x2) formed at each of the bottom ends of the first and second branched magnetic path legs A and Axe2x80x2 (B and Bxe2x80x2) and arranged in the direction of advance at intervals of xc2xd of the dot pitch P. Here, each branched magnetic path leg may be formed with only one pole tooth, but in the event of several, the spatial phase held with respect to the closest dots of the platen dots D is the same. Further, the interval between the first branched magnetic path leg A (B) and second branched magnetic path leg Axe2x80x2 (Bxe2x80x2) is set so that the spatial phases with respect to the closest dots are shifted in the direction of advance by exactly P/2. Further, the interval between the second branched magnetic path leg Axe2x80x2 and the first branched magnetic path leg B is set so that the spatial phases with respect to the closest dots are shifted in the direction of advance by exactly P/4.
The movable member 20 has a pressurized air ejection port and floats slightly above the surface of the platen 10 by blown pressurized air. As shown in FIG. 30A, if a B-phase current of the illustrated polarity is flown through only the terminals of the first and second B-phase excitation coils CB and CBxe2x80x2 of the second yoke Y2, not only the bias magnetic flux due to the permanent magnet M, but also the alternating magnetic flux due to the second excitation coil CBxe2x80x2 are superposed and strengthened to generate a concentrated magnetic flux portion xcex1 in the air gap between the pole teeth KBxe2x80x2 of the second branched magnetic path leg Bxe2x80x2 and the closest dots D1 and D2 and strongly magnetically draw the pole teeth KBxe2x80x2 to the closest dots D1 and D2. Also, an alternating magnetic flux is applied to the pole teeth CB of the first branched magnetic path leg B in a direction canceling out the bias magnetic flux, so an extinguished magnetic flux portion xcex2 is formed. On the other hand, the magnetic flux comprised of the concentrated magnetic flux from the second branched magnetic path leg Bxe2x80x2 of the second yoke Y2 branched via the inside of the platen 10 passes through the first and second branched magnetic path legs A and Axe2x80x2 of the first yoke Y1, but the pole teeth KA of the first branched magnetic path leg A are delayed in the direction of advance by exactly P/4 with respect to the closest dots D15 and D14. Therefore, the closest dots D15 and D14 pull the pole teeth KA in the direction of advance by one branched magnetic flux and the pole teeth KAxe2x80x2 of the second branched magnetic path leg Axe2x80x2 proceed in the direction of advance by exactly P/4 with respect to the closest dots D10 and D9 due to the other branched magnetic flux. Accordingly, the closest dots D10 and D9 pull the pole teeth KAxe2x80x2 in a direction opposite to the direction of advance. Therefore, the thrust in the direction of advance and the pullback force in the reverse direction match each other perfectly and the first yoke Y1 as a whole is balanced. That is, a thrust branched magnetic flux portion xcex4 is generated in the air gap between the pole teeth KA of the first branched magnetic path leg A and the closest dots D15 and D14, while a pullback force branched magnetic flux portion xcex3 is generated in the air gap between the pole teeth KB of the second branched magnetic path leg B and the closest dots D10 and D9, so the first yoke Y1 itself becomes a stable point of the magnetic attraction potential.
Next, as shown in FIG. 30B, if an A-phase current of the illustrated polarity is supplied to only the terminals of the first and second A-phase excitation coils CA and CAxe2x80x2 of the first yoke Y1, the air gap between the pole teeth KA of the first branched magnetic path leg A and the closest dots D15 and D14 switches from what had been the thrust branched magnetic flux portion xcex4 immediately before to the concentrated magnetic flux portion xcex1 comprised of the bias magnetic flux plus the alternating magnetic flux from the second excitation coil 4 superposed, while the pole teeth KAxe2x80x2 of the second branched magnetic path leg Axe2x80x2 switch from the pullback branched magnetic flux portion xcex3 to the extinguished magnetic flux portion xcex2, so the closest dots D15 and D14 strongly magnetically draw the pole teeth KA and advancing thrust occurs at the movable member 20. On the other hand, a branched magnetic flux to form the concentrated magnetic flux at the first branched magnetic path leg A of the first yoke Y1 through the inside of the platen 10 passes through the first and second branched magnetic path legs B and Bxe2x80x2 of the second yoke Y2. The pole teeth KB of the first branched magnetic path leg B switch from the extinguished magnetic flux portion xcex2 to the thrust branched magnetic flux portion xcex4, while the pole teeth KBxe2x80x2 of the second branched magnetic path leg Bxe2x80x2 switch from the concentrated magnetic flux portion xcex1 to the pullback branched magnetic flux portion xcex3. Therefore, due to the switching of the two-phase current, the movable member 20 advances by exactly P/4. If including the excitation patterns of FIGS. 30C and 30D, with a two-phase current, there are four excitation patterns of the excitation coils, so by one round of the excitation patterns, the movable member 20 advances four times and proceeds by exactly one pitch worth of distance. In the process of the switching of the two-phase current, a thrust force is generated at the pole teeth moving from the thrust branched magnetic flux portion xcex4 to the concentrated magnetic flux portion xcex1.
To realize a planar linear motor having a movable member which moves planarly (two-dimensionally) in the Y-axial and Y-axial directions using such a Sawyer linear motor, for example, as seen in Japanese Unexamined Patent Publication (Kokai) No. 9-261944, as shown in FIG. 31 and FIG. 32, there are provided a platen 10 formed on the platen surface with substantially square-top platen dots D arranged in a matrix and a composite movable member comprised of X-axis movable members 20 having stripe-shaped projecting pole teeth KA and KAxe2x80x2 (KB and KBxe2x80x2) parallel to the Y-axis and providing drive in the X-axial direction and Y-axis movable members 20Y having stripe-shaped projecting pole teeth KA and KAxe2x80x2 (KB and KBxe2x80x2) parallel to the X-axis and providing drive in the Y-axial directionxe2x80x94all connected by a support plate 30 in an in-planar perpendicular relationship.
On the other hand, the platen serving as the stator essential for the planar linear motor uses a pure iron plate formed by a single block material as the platen body and has a backing reinforcing plate of a thick steel plate bonded to the back surface by welding. The surface of the platen body is formed with platen dots arranged in a matrix by cutting. A resin etc. is filled in the lattice-like grooves between the dots, then the surface is flattened by precision polishing. The backing reinforcing plate is required for preventing warping of the platen surface and ensuring flatness when precision polishing the platen surface. If a pure iron platen body is used, however, an eddy current naturally occurs due to the magnetic flux passing through the inside of the platen body of this uniform continuous plate, so the AC magnetizing characteristic is poor and the power loss (iron loss) large and therefore it is difficult to obtain a high speed, high thrust force movable member and a large current capacity is required. The higher the frequency the driving periodic current (current pulse) is made and the higher the speed of the advance, the more rapidly the thrust force falls and the worse the efficiency (speedxc3x97thrust force/power consumption) becomes.
Therefore, the present inventors got the idea of suppressing the occurrence of the eddy force at the platen body and realizing a high speed, high thrust, and high efficiency planar linear motor by using a stacked member comprised of a plurality of magnetic sheets (for example, a thickness of not more than 1 mm), and using the parallel sheet edge surface side of the stacked member as the platen surface. If using a stacked member of sheets as the platen body, since an eddy current does not easily pass through the stacked interfaces (joined surfaces) of the magnetic sheets, the occurrence of an eddy current can be suppressed, so it is possible to realize a high speed, high thrust, high efficiency planar linear motor.
Here, the magnet flux in the stacked member is refracted or blocked at the joined surfaces and conversely the magnetic resistance is high, so it was thought that it was not actually possible to form a magnetic circuit for an advancing magnetic flux along the normal direction of the joined surfaces and that advance of the monoaxial movable member in the direction perpendicular to the sheet edge direction was impossible. The present applicant, however, as disclosed in Japanese Patent Application No. 2000-56721, found that in the case of an n-phase drive current and a movable member having an electrode tooth pattern comprised of 2n number of electrode teeth, if the electrode teeth are arranged laterally in a line in an equivalent spatial phase relationship with respect to the closest dots arranged in the sheet edge direction of the magnetic sheets, the 2n number of electrode teeth are arranged at staggered positions within one dot pitch (P) in the normal direction of the joined surfaces of the magnetic sheets, and the spatial phase with respect to the closest dots arranged in the normal direction differ by increments of exactly the difference in spatial phase (P/2n), the movable member having such an electrode tooth pattern moves by a crawling advancing motion in the direction perpendicular to the sheet edge direction.
When using a stacked member comprised of a large number of magnetic sheets stacked together as the platen body, aside from the advantages in performance of the linear motor, it is possible to obtain advantages in manufacturing such as the ability to form the platen dots on the magnetic sheets not by a cutting step, but by etching or a punch press. Since not the stacked surface, but the parallel sheet edge surface of the stacked member (sheet edge lateral cross-section) is used as the platen surface, however, to obtain a platen surface of a size of 1 mxc3x971 m using strip-shaped magnetic sheets of a thickness of 1 mm, over 1000 sheets become necessary. The stacked thickness becomes more than 10 times greater than the width of the strip-shaped magnetic sheets (corresponding to thickness of platen body) in this stacked structure. In view of this, it is necessary to give full consideration to maintaining the shape of the stacked member itself and to prevent deformation.
Therefore, in view of these problems, a first object of the present invention is to provide for practical use a platen for a planar linear motor using a stacked member of magnetic sheets by realizing a binding structure of the magnetic sheets. A second object of the present invention is to provide for practical use a platen for a planar linear motor using a stacked member of magnetic sheets by realizing a structure restricting torsional deformation of the stacked member. A third object of the present invention is to provide for practical use a platen for a planar linear motor using a stacked member of magnetic sheets by realizing reduced weight of the stacked member.
A platen for a planar linear motor according to the present invention is provided with a platen body using a stacked member comprised of a large number of magnetic sheets aligned and stacked together and having a large number of platen dots formed in a two-dimensional array at one parallel sheet edge surface side (platen surface) of the stacked member. It has a connecting beam member supporting said stacked member at regular discrete positions in the sheet edge direction at the other parallel sheet edge surface side of the stacked member and a binding means for binding the magnetic sheets between the other parallel sheet edge surface side and the connecting beam members.
According to this configuration, since the stacked member is bound in a perpendicular direction to the sheet edge direction at the other parallel sheet edge surface by connecting beam members serving as the binding means, breakdown or deformation of the stacked member can be suppressed. Further, since one parallel sheet edge surface side of the stacked member is free, the flatness as a platen surface can be secured. Further, the binding means do not occupy the entire area of the other parallel sheet edge surface. The areas between the connecting beam members are non-bound areas. Therefore, it is possible to suppress to a maximum the working deformation or strain etc. of the magnetic sheets accompanying the binding step from reaching the platen surface side. By reducing the thickness of the stacked member (width of magnetic sheets), it is possible to achieve a lower cost and lighter weight of the platen. Of course, the connecting beam members themselves function as support members of the stacked member, so it is possible to ensure the retention of the shape of the stacked member even at the time of actual use such as at the time of movement when carrying or suspending the movable member.
In general, there is a tradeoff between the working deformation etc. of magnetic sheets accompanying the binding step reaching the platen surface and the reduction of thickness of the stacked member. Considering the former problem, the proximate cause is the initial stress occurring due to the mechanical working of the magnetic sheets themselves forming the stacked member in the binding step. Therefore, to realize reduced weight of the platen by reducing the width of the magnetic sheets, it is necessary to employ binding means resistant to initial stress at the other parallel sheet edge surface side of the stacked member. For example, when employing binding means using holes passing through the stacked member in the stacking direction as binding holes and press-fitting through rods through the same, the compression stress extends from around the binding holes at the time of press-fitting the through rods. Further, to provide the binding holes, it is necessary to increase the width of the magnetic sheets by the amount of the binding margin. Therefore, use of binding means accompanied with stress is not suitable for reducing the thickness of a stacked member.
The present inventors engaged in intensive research into a binding means resistant to the occurrence of working stress and as a result took note of a binding means using injection of a fluid hardening (solidifying) material. Joints made using brazing or welding can also in a certain sense be called binding means using a fluid hardening material (filler material), but at the time of brazing or welding, joining is difficult without preheating the areas scheduled for joining of the base material (stacked member) to a high temperature, so heat affected areas and deformation under contraction due to rapid cooling occur in the base material, that is, the magnetic sheets, the insulating film of the magnetic sheets deteriorate under the heat, and residual stress or warping or other deformation arise.
Therefore, as the binding means, male parts or female parts are formed along a perpendicular direction of the other parallel sheet edge surface of the stacked member and use is made of joints of a fluid hardening material having molded connecting parts fastening with the same and molded joining parts connecting with the molded connecting parts and gripping parts of the connecting beam members. The binding means according to the present invention simultaneously provide a binding function of binding together the large number of magnetic sheets of the stacked member and a connecting function of connecting the stacked member and connecting beams serving as support members. The molded connecting parts fastening with the male parts or the female parts of the stacked member by filling and hardening provide the binding function, while the molded joining parts gripping parts of the connecting beam members provide the connecting function. The male parts or the female parts are formed at the stacked member in a step before injecting the fluid hardening material so as to secure areas of an anchor action or filled with the fluid hardening material and thereby attain the binding function. Further the gripping joining structure is used so as to attain a locking type fastening action. This is particularly beneficial when supporting the platen hanging down. As the male part, a stacked ridge comprised of projections provided at the magnetic sheets may be used. The projections may be formed with through holes. It is possible to form the edges of the projections serrated in order to improve the filling power. Further, the female part may be a groove formed by stacking notches provided in the magnetic sheets. Male parts and female parts may be used together as well. Further, binding holes or cut slits may also be used. Since the binding means are made of a fluid hardening material, before the step of injecting the fluid hardening material, it is sufficient to form through holes etc. for forming molded joining parts in addition to the male parts or female parts at the connecting beam members and other support side as shaping parts of the mold. Further, since it is possible to form a plurality of molded joining parts at one time simultaneously with the molded connecting parts by injecting the fluid hardening material, a reduction in cost can be expected through a reduction in the number of manufacturing steps. The plurality of molded connecting parts can be compared with the branching stems from fibrous underground plants, but since the attachment force of the male parts or female parts and the molded connecting parts is secured by the attachment area, even when the platen is supported hanging down, since the supporting force is dispersed among the plurality of molded joining parts, there is resistance to separation of the male parts or female parts and the molded joining parts and both a binding function and connecting function can be achieved.
Here, the selection of the fluid hardening material is important. Even an adhesive can provide a considerable attachment force, but even if the bond strength is large, if the strength of the solidified adhesive is weak, that solidified portion will easily break, so use of a molten resin material or a molten metal material is preferable. When using a molten metal material, even if the melt temperature is high, when the molten metal material is injected, it is rapidly cooled and hardened, so the problem of thermal strain is not that serious. However, it is preferable to inject the melt while cooling the stacked member. It is also possible to cool just the platen surface side. Further, it is possible to use a filler material melting at a comparatively low temperature. Preferably, an aluminum alloy having a melting point of 200 to 400EC (brand name Alumite) or solder is suitable. In the case of a low temperature solder, the casting becomes incomplete at heat sinks of the stacked member and there is a danger of formation of islands. In the case of a molten metal material, the mechanical strength of the molded parts rises, but fusion bonding with the male parts or female parts is difficult due to compatibility with the base material and the bond strength may not be able to be secured.
Therefore, to supplement the bond strength, it is possible to improve the anchor effect by making joint use of mechanical locking mating parts. That is, in the case of a male part, by using a projecting ridge and making its lateral cross-section for example broad in the front end and narrow in the base, the molded connecting part is naturally formed as a female part, so locking mating parts are formed and the anchor effect can be raised. Further, in the case of a female part, by using a groove and making its lateral cross-section for example narrow in opening and broad in interior, the molded connecting part is naturally formed as a male part, so locking mating parts are formed and the anchor effect can be raised. Use of even molten metal materials with a poor compatibility in fusion bonding becomes possible and the freedom of selection of the molten metal material is increased. Rather, a poor compatibility in fusion bonding means greater resistance to formation of heat affected areas in the magnetic sheets (silicon steel sheets etc.) and deformation under contraction can be suppressed.
In this way, by employing an injection type joint of a fluid hardening material, it is possible to simultaneously achieve a binding function and a connecting function. This is however not limited to just the stacked member and the connecting beam members. It is also possible to interpose various auxiliary members between the stacked member and the connecting beam members. The stacked member is supported by the connecting beams, but when configuring the platen by just these, if an outside force or inertial force acts laterally on the stacked member during transport of the platen etc., torsion will occur in the magnetic sheets, the sheet edges will bend due to the torsion, the advancing magnetic fields between staggered electrode teeth of the movable member will become asynchronous, and advance may become impossible. In particular, torsion becomes more serious the smaller the thickness or width of the magnetic sheets.
Therefore, it is preferable to provide an outside frame abutting against at least the two side surfaces of the stacked member in the stacking direction and clamping the stacked member. To suppress deformation of the outside frame itself, it is effective to place a backing plate (sheet) against the other parallel sheet edge surface of the stacked member. The backing plate functions as a spacer for defining the distance between facing side plates of the outside frame. Further, the backing plate provides support at the areas not facing the connecting beams in the other parallel sheet edge surface, so can directly prevent deformation of the stacked member. In addition, since the other parallel sheet edge surface is placed against the flat surface of the backing plate, if the width dimensions of the magnetic sheets are controlled to a high precision, the flatness of the platen surface can be secured, use of strip-shaped magnetic sheets having platen dot projections becomes possible, and the step of forming the platen dots on the stacked member after stacking can be eliminated. Note that the platen body and the outside frame may suitably form a box structure.
On the other hand, as a structure enabling prevention of deformation of the stacked member without using a backing plate, there is a stacked member having bonding layers interposed between the adjoining magnetic sheets. Of course, use of a backing plate as well is also possible. As the bonding layers, coated layers of an epoxy resin or other adhesive are suitable. It is possible to repeatedly stack magnetic sheets while interposing the bonding layers so as to obtain the stacked member. There is no problem even with some unevenness of coating. To control the thickness of the bonding layers, for example, after coating the adhesive on a magnetic sheet surface, cut pieces of the magnetic sheets or other spacer materials may be scattered on it, then the next magnetic sheet pressed down to push out the excessive adhesive in the gap. It is also possible to scatter cut pieces of the magnetic sheets or other spacer materials on a magnetic sheet formed with through holes or notches, overlay the next magnetic sheet, then, after preparing a stacked member of the necessary number of sheets, immersing it in an adhesive to fill the gaps between the sheets by the adhesive through the through holes or notches and thereby form the bonding layers all at once. The through holes are simultaneously filled, so it is possible to prevent deformation of the stacked member and secure the required strength. Further, the through holes or notches perform the function of concentrating the advancing magnetic paths at the parts of current resistance suppressing eddy current or the platen surface side. This contributes to the higher performance of the linear motor.
Note that when employing an injection type joint of a molten metal material for a stacked member having bonding layers of an adhesive interposed in it, the molten metal material hardens instantaneously. There is of course no problem with deterioration of the adhesive. Rather, quick drying and curing of the adhesive can be expected due to the surplus heat and the drying and aging step can be simplified.
The platen dots may be formed on the parallel sheet edge surface of the stacked member by shape-cutting electrodischarge machining or may be formed by etching. Here, in the case of a stacked member without bonding layers, the working fluid is liable to penetrate deep into the clearances between the sheets. In the case of a stacked member with bonding layers, when the degree of adhesion is high, the bonding layers function as masks against the working fluid, so deep penetration of the working fluid can be prevented.
Now, explaining the specific binding means below, when using a backing plate, the platen body is comprised of the stacked member and a backing plate placed against the parallel sheet edge surface of the same, a plurality of first through holes discretely arranged in lines longitudinally are formed across the strip-shaped portions facing the grooves serving as female parts of the backing plate, each connecting beam member is provided with a bent side end to be placed against the backing plate, a plurality of second through holes discretely arranged in lines longitudinally are formed in the beam longitudinal direction of the bent side ends, the molded connecting parts are made male molded parts formed by filling the grooves, and the molded joining parts are made rivet-shaped molded parts formed by filling the first and second through holes. It is possible to use a fluid hardening material such as an aluminum alloy to achieve an anchor effect. The grooves may be dovetail grooves or may be partially circular in lateral cross-section. Lateral cross-section inverted T-shaped notches, lateral cross-section inverted L-shaped notches, lateral cross-section F-shaped notches, lateral cross-section S-shaped notches, and other notches forming grooves in lateral cross-section may be formed in the magnetic sheets by punching before stacking, so the female mating parts can be naturally formed. The invention is not limited to the case of formation of openings or slits etc. of female parts in the other parallel sheet edge surface in the above way. It is also possible to provide through holes in the stacking direction for serving as runners at positions close to the other parallel sheet edge surface and form openings or slits exposed at the other parallel sheet edge surface discretely connected to these through holes.
When not using a backing plate, the binding means may also be a welded joint formed by laser beam welding the abutting surfaces of the side surface of a connecting beam member and the parallel sheet edge surface along the same. The welding is not arc welding or gas welding etc., but is instantaneous welding restricting the welding zone, so thermal strain etc. of the stacked member can be prevented. However, it is better to perform the laser beam welding while cooling the platen surface side. Since a temperature gradient is formed, it is possible to prevent the thermal strain from reaching the platen surface.
Here, when the side end of a connecting beam member is the plate edge, if the side end surface is placed against the parallel sheet edge surface, the welded joint becomes a T-shaped joint, so the range of support of the stacked member is limited to the thickness dimension of the plate. Further, when securing a balance of strength in connection by welding at both the front and back sides of the plate, the fillet welds become close to each other across the distance of the plate thickness, so the thermal strain becomes superposed and becomes larger at the abutting portions. Therefore, it is preferable to use a connecting beam member having an inverted L-shaped bend side end and place the outside surface of that bent side end against the parallel sheet edge surface. The support becomes broader in range and stabler and the fillet welds are separated by the width of the bent side end, so it is possible to prevent superposition of thermal strain etc.
When the compatibility of the stacked member and the connecting beams in laser beam welding is poor, spacers with good compatibility are used. That is, each connecting beam member is comprised of a beam body having a bent side end and a long spacer placed against and fastened with the outside surface of that bent side end. The binding means is made a welded joint formed by laser beam welding the abutting parts of the long spacer and the other parallel sheet edge surface along the same.
In the case of the injection type joint of the fluid hardening material explained above, the functions are split among the molded connecting part and molded joining part, but it is also possible for substantially the same portion to provide both functions. For example, the stacked member has grooves formed along a perpendicular direction in the other parallel sheet edge surface, each connecting beam member is provided with a beam body having a bent side end and a long male part provided by fastening or integrally with the outside of the bent side end, and the binding means is made a fluid hardening material formed by filling the clearances when loosely fitting together the long male part and the groove. When the fluid hardening material is a fusion bonding molten metal, the molded connecting part formed along the clearances exhibits the binding function and provides the connecting function by fusion bonding with the long male part. With a nonfusion bonding fluid hardening material, however, while a binding function can be obtained, a connecting function cannot be obtained. By making the lateral cross-section of the groove for example narrow in opening and broad in interior and making the lateral cross-section of the long male part for example broad in front end and narrow in base, a locking action is exhibited, so a connecting function is obtained. In this case, of course the end of the long male part is inserted into the end opening of the groove. Conversely, actions and effects similar to the above are exhibited even by a structure providing for example a long male part of a cross-section broad in the front end and narrow in the base at the stacked member, forming for example a groove of a cross-section narrow in opening and broad in interior at the beam body side, loosely fitting together the long male part and the groove, and filling the fluid hardening material in the clearances. A structure providing a long male part or groove at the beam body side in this way, however, requires the fastening of separate members or cutting work, which leads to higher manufacturing costs.
Therefore, to form a simple binding means not using a backing plate, it is preferable that the female part of the stacked member be a groove, the connecting beam member have a bent side end, the bent side end have a plurality of through holes discretely arranged longitudinally in a line in the beam longitudinal direction, the molded connecting part be a male molded part formed by filling the groove, and the molded joining part be a rivet-shaped molded part formed by filling the through holes. It is even better if the groove is narrow in opening and broad in interior in lateral cross-section. The fastening force of the rivet-shaped molded part is strong and the locking action is high, so loosening will not easily occur even if vibration occurs in the platen during operation.
Conversely, the stacked member may be formed with a projecting ridge as a male part, each connecting beam member may have a groove having a plurality of through holes discretely arranged in a line longitudinally along the beam longitudinal direction in the groove bottom, the molded connecting part may be a female molded part formed by filling the remaining clearance in the groove when the groove accommodates the projecting ridge, and the molded joining part may be a rivet-shaped molded part formed by filling the through holes. It is even better if the projecting ridge is made broad in front end and narrow in base in lateral cross-section.
Further, it is possible to employ a configuration in which the female part is narrow in opening and broad in interior in lateral cross-section, each connecting beam member has a plurality of notches formed discretely along the beam longitudinal direction of its side end surface, the molded connecting part is a male molded part formed by filling the remaining clearance when the side end surface is made to abut against the bottom surface of the groove, and the molded joining part is a rivet-shaped molded part formed by the overflow of the material from the opening of the groove. The locking of the stacked member and molded connecting part need not be continuous and may be just discrete as well.
Further, it is possible to employ a configuration in which the female part of the stacked member is a first groove narrow in opening and broad in interior in lateral cross-section, each connecting beam member has a bent side end, the bent side end has a second groove formed at its outside surface along the beam longitudinal direction and narrow in opening and broad in interior in lateral cross-section, and each binding means is a pegged dual bulging end molded part formed by filling the first and second grooves in a mated state. The first groove of the stacked member and one bulging portion of the pegged dual bulging end molded part provide the binding function and a locking function, while the other bulging portion of the pegged dual bulging end molded part and the connecting beam members provide a locking and connecting function. This corresponds to a so-called molded rivet.